The Fawlty Towers star’s husband, actor Timothy West, talks about Ms Scales’ condition in a new More4 documentary called Grand Canal Journeys which is due to be broadcast later this month.

Mr West, who is also a Norwich Playhouse patron and recently joined the cast of Eastenders, tells the documentary that 81-year-old Ms Scales suffers from “a sort of mild Alzheimer’s”, the Radio Times has said.

The couple have been navigating Britain’s canals on a slow boat ever since they first sparked a passion for the hobby when they borrowed a friend’s boat for a fortnight in the 1970s.

Mr West tells the programme, in which the pair embark on four canal journeys across Britain, that the journeys are perfect for his wife because of the difficulties with her memory.

“She can’t remember things very well, but you don’t have to remember things on the canal,” he says.

“You can just enjoy things as they happen - so it’s perfect for her.”

Ms Scales, perhaps best known for playing Sybil Fawlty in the BBC comedy but who has also notched up an array of other stage and screen roles, says that she was determined not to let the condition keep her from the stage.

“I always say I want to die on the eighth curtain call,” she says.

“Eight will mean the show’s been rather a success. I just hope I’m somewhere near the middle and have been reasonably good in the part.”

Mr West, 79, and Ms Scales have been supporters of Norwich Playhouse since it was first founded in the 1990s.

They helped raise funds to equip and open the building and Mr West starred in Norwich Playhouse’s first ever show, Jumpers by Tom Stoppard.

Ms Scales last performed at Norwich Playhouse in 2009 in Gertrude’s Secret, a drama about an older woman who is suffering domestic abuse.

Norwich Playhouse general manager Caroline Richardson said: “Norwich Playhouse is very lucky to have both Prunella Scales and Timothy West as our patrons, and as always we wish them both very well.”

Ms Scales has also been a supporter of Norwich Puppet Theatre, and was among the celebrities to create a puppet that was auctioned off at a special fundraiser to mark the theatre’s 30th anniversary.